1. Field of the Invention
The present invention relates generally to a magnetron, and a microwave oven and a high-frequency heating apparatus, each equipped with the magnetron and more particularly, to a magnetron in which the arrangement of magnets applying magnetic flux to the activating space of the magnetron are improved, and the shapes of upper and lower yokes are changed to correspond to the improvement of the arrangement, and a microwave oven and a high-frequency heating apparatus each equipped with the same.
2. Description of the Related Art
A construction of a conventional magnetron is described with reference to an accompanying drawing. As illustrated in FIG. 1, in the conventional magnetron, a plurality of vanes 102 that constitute an anode together with an anode cylinder 101 are radially arranged at regular intervals to form resonance circuits, an antenna 103 is connected to one of the vanes 102 to transmit harmonics to the outside, and the vanes 102 are alternately connected to each other by two pairs of strip rings 108. Additionally, a cathode including a filament 106 that is fabricated in the form of a coil spring to emit thermions is disposed along the central axis of the anode cylinder 101. An activating space 107 is formed between the filament 106 and the radially inner ends of the vanes 102. Meanwhile, an upper shield 109a and a lower shield 109b are attached to the top and bottom of the filament 106, respectively. A center lead 110 is fixedly welded to the upper shield 109a with its middle portion passed through the through hole of the lower shield 109b and the filament 106. A side lead 111 is welded to the bottom of the lower shield 109b. The center lead 110 and the side lead 111 are electrically connected to terminals of an external power source (not shown) and consequently form a closed electric circuit, so an electric field is generated in the activating space 107. Meanwhile, an upper permanent magnet 112 and a lower permanent magnet 113 are provided above and below the anode, respectively, with the opposite magnetic poles of the upper and lower permanent magnets 112 and 113 facing each other. An upper pole piece 117 and a lower pole piece 118 are provided to carry magnetic flux generated by the permanent magnets 112 and 113 to the activating space 107. The above-described elements are enclosed by an upper yoke 114 and a lower yoke 115. A closed magnetic circuit has component elements that are arranged in the order of the upper permanent magnet 112, the upper pole piece 117, the activating space 107, the lower pole piece 118, the lower permanent magnet 113, the lower yoke 115, the upper yoke 114 and the upper permanent magnet 112. Cooling fins 116 are provided to discharge heat generated in the anode through the lower yoke 115 to the outside by connecting the high temperature anode cylinder 101 with the lower yoke 115, because the anode cylinder 101 is heated by collisions between the thermions and the anode, that is, the radially inner ends of the vanes 102. Reference numerals 104 and 105 designate an upper shield cup and a lower shield cup, respectively, to keep the activating space vacuumized. FIG. 2 is a perspective view of FIG. 1.
With the above-described construction of the magnetron, when external power is applied to the filament 106, the filament 106 is heated by operating current applied to the filament 106, and thermions are emitted from the filament 106. A group of thermions formed by continuously emitted thermions alternately impart a potential difference to each neighboring pair of vanes 102 while coming in contact with the radially inner ends of the vanes 102 after undergoing combined rectilinear and rotational movement under the influence of electric and magnetic fields generated in the activating space. Accordingly, oscillations are continuously generated in the resonance circuits of the anode, and harmonics corresponding to the rotation speed of the group of thermions are generated and transmitted to the outside through the antenna 103.
In general, the magnetrons are widely used as component parts in home appliances, such as microwave ovens, as well as in industrial applications, such as high-frequency heating apparatuses, particle accelerators and radar units.
In the meantime, in the conventional magnetron, the permanent magnets are provided above and below the anode in consideration of the uniformity and symmetry of magnetic flux across the activating space of the magnetron, so the height and volume of the magnetron and the lengths of parts (such as the center lead, the side lead, the antenna, the upper and lower shield cups and ceramic (not shown)) made of expensive materials are increased, thus increasing the weight and manufacturing cost of the magnetron.
Meanwhile, in the conventional magnetron, the permanent magnets come in tight contact with the anode heated by the absorption of thermions to suppress an increase in the volume of the magnetron. Hence, the demagnetization of the permanent magnets is caused by the heating of the permanent magnets, and the size of the magnetron is increased in consideration of the decrease of the oscillation efficiency, thus reducing the oscillation efficiency of the magnetron and increasing the weight and manufacturing cost of the magnetron, respectively. Therefore, there have been many attempts to suppress the demagnetization of permanent magnets.